The switching power supply is a power electronic product that uses power semiconductor devices and integrates power conversion technology, electronic electromagnetic technology, and automatic control technology. Because of its advantages of low power consumption, high efficiency, small size, light weight, stable operation, safe and reliable, and wide voltage regulation range, it is widely used in computers, communications, electronic instruments, industrial automation, national defense and household appliances. field. However, the switching power supply has poor transient response and is prone to electromagnetic interference, and the EMI signal occupies a wide frequency range and has a certain amplitude. These EMI signals pollute the electromagnetic environment through conduction and radiation, causing interference to communication equipment and electronic equipment, thus limiting the use of switching power supplies to some extent.
Causes of electromagnetic interference generated by switching power supplies
Electromagnetic interference (EMI) is a performance impairment caused by an unexpected electromagnetic disturbance in an electronic system or sub-system. It consists of three basic elements: the source of interference, that is, the device that generates electromagnetic interference energy; the way of combining, that is, the path or medium that transmits electromagnetic interference; the sensitive device, that is, the device, device, subsystem or device that is damaged by electromagnetic interference or system. Based on this, the basic measures for controlling electromagnetic interference are: suppressing the source of interference, cutting off the path of mismatch, and reducing the response of sensitive equipment to interference or increasing the level of electromagnetic sensitivity.
According to the working principle of the switching power supply, the switching power supply first rectifies the commercial frequency alternating current into direct current, and then inverts into high frequency alternating current, and finally rectifies and filters the output to obtain a stable direct current voltage. In the circuit, the power transistor and the diode mainly work in the state of the switch tube, and work in the microsecond order; during the turn-on and turn-off process of the triode and the diode, the current changes greatly during the rising and falling time, and the RF energy is easily generated. Source of interference. At the same time, potential electromagnetic interference is also formed due to the leakage inductance of the transformer and the spike caused by the reverse recovery current of the output diode.
Switching power supplies typically operate at high frequencies with frequencies above 02 kHz, so their distributed capacitance is not negligible. On the one hand, the insulating sheet between the heat sink and the collector of the switch tube has a large contact area and a thin insulating sheet. Therefore, the distributed capacitance between the two cannot be ignored at high frequencies, and the high-frequency current flows through the distributed capacitor. On the heat sink, it flows to the chassis ground to generate common mode interference. On the other hand, there is a distributed capacitance between the primary and secondary of the pulse transformer, which can directly confuse the primary winding voltage to the secondary winding. Common mode interference occurs on the two power lines that make DC output.
Therefore, the interference sources in the switching power supply are mainly concentrated in voltage and current variations, such as switching tubes, diodes, high-frequency transformers and other components, as well as AC input and rectifier output circuit parts.
Measures to suppress electromagnetic interference of switching power supply
Generally, switching power supply EMI control mainly uses filtering technology, shielding technology, sealing technology, grounding technology, and the like. EMI interference is divided into conducted interference and radiated interference according to the propagation route. The switching power supply is mainly conducted interference, and the wide frequency range is about 10 kHz to 30 MHz. The countermeasures for suppressing conducted interference are basically solved in three frequency bands of 10 kHz to 150 kHz, 150 kHz to 10 MHz, and 10 MHz or more. The main interference is mainly in the range of 10 kHz to 150 kHz, and is generally solved by a general-purpose LC filter. The common mode interference is mainly in the range of 150 kHz to 10 MHz, and is usually solved by a common mode rejection filter. The countermeasures for the frequency band above 10 MHz are to improve the shape of the filter and to take electromagnetic shielding measures.
AC input EMI filter
There are usually two ways in which the interference current is transmitted on the wire: the common mode and the differential mode. Common mode interference is the interference between the carrier fluid and the earth: the interference size and direction are the same, and exist in any relative ground of the power supply, or between the neutral line and the earth, mainly generated by du/dt, and the di/dt also produces a certain total. Mode interference. The differential mode interference is the interference between the carrier fluids: the interference is equal in magnitude and opposite in direction, and exists between the power phase line and the neutral line and between the phase line and the phase line. When the interference current is transmitted on the wire, it can be either common mode or differential mode; however, the common mode interference current can only interfere with the useful signal after it becomes the differential mode interference current.
The above two types of interference exist on the AC power input line, usually low-band differential mode interference and high-band common mode interference. Under normal circumstances, the differential mode interference amplitude is small, the frequency is low, and the interference caused is small; the common mode interference amplitude is large, the frequency is high, and the radiation can be generated by the wire, and the interference caused is large. If an appropriate EMI filter is used at the input end of the AC power source, electromagnetic interference can be effectively suppressed. The basic principle of the power line EMI filter is shown in Figure 1. The differential mode capacitors C1 and C2 are used to short-circuit the differential mode interference current, while the intermediate connection grounding capacitors C3 and C4 are used to short-circuit the common mode interference current. The common mode choke is composed of two coils that are equal in thickness and wound in the same direction on a magnetic core. If the magnetic coupling between the two coils is very tight, the leakage inductance will be small and the difference will be within the power line frequency range.
The mode reactance will become very small; when the load current flows through the common mode choke, the magnetic lines of force generated by the coils connected in series on the phase line are opposite to the lines of magnetic force generated by the coils connected in series on the center line, and they are in the core. Cancel each other out. Therefore, even in the case of a large load current, the core does not saturate. For the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will exhibit a large inductance, thereby attenuating the common mode interference signal. Here, the common mode choke coil is made of a ferrite magnetic material having a high magnetic permeability and a good frequency characteristic.
Improve the switching waveform with the absorption loop
During the turn-on and turn-off of the switch or diode, due to the transformer leakage inductance and line inductance, the diode storage capacitor and distributed capacitance, it is easy to generate a spike voltage at the collector, emitter and diode of the switch. The RC/RCD absorption loop is usually used, and the RCD surge voltage absorption loop is shown in Figure 2.
When the voltage on the absorption loop exceeds a certain range, the devices are quickly turned on, thereby venting the surge energy and limiting the surge voltage to a certain amplitude. The saturable core coil or the microcrystalline magnetic bead is connected in series on the positive electrode lead of the collector of the switch tube and the output diode, and the material is generally cobalt (Co). When the normal current is passed, the core is saturated, and the inductance is small. Once the current flows in the opposite direction, it will generate a large back EMF, which effectively suppresses the reverse surge current of the diode VD.
Switching frequency modulation
The frequency control technique is based on the energy of the switching interference, which is mainly concentrated on a specific frequency and has a large spectral peak. If these energy can be dispersed over a wider frequency band, the purpose of lowering the peak of the disturbance spectrum can be achieved. There are usually two methods of processing: the random frequency method and the modulation frequency method.
The random frequency method adds a random perturbation component to the circuit switching interval, so that the switching interference energy is dispersed in a certain frequency band. Studies have shown that the switching interference spectrum changes from the original discrete spike interference to continuous distributed interference, and its peak value is greatly reduced.
The modulation frequency method adds a modulated wave (white noise) to the sawtooth wave, forms a sideband around the discrete frequency band in which the interference occurs, and expands the discrete frequency band modulation of the interference into a distributed frequency band. In this way, the interference energy is spread over these distributed frequency bands. This control method can well suppress the interference during turn-on and turn-off without affecting the operating characteristics of the converter.
Soft switching technology
One of the interferences of the switching power supply is the du/dt from the on/off of the power switch. Therefore, reducing the on/off of the power switch on/off is an important measure to suppress the interference of the switching power supply. The soft switching technology can reduce the on/off of the on/off of the switch.
If a small resonant component such as an inductor or a capacitor is added to the switching circuit, an auxiliary network is formed. Leading the resonance process before and after the switching process, the voltage is first reduced to zero before the switch is turned on, so that the phenomenon of voltage and current overlap during the opening process can be eliminated, and the switching loss and interference can be reduced or even eliminated. This circuit is called a soft switching circuit. .
According to the above principle, two methods can be used, that is, the current is zero before the switch is turned off, and no loss or interference occurs when the switch is turned off. This shutdown mode is called zero current shutdown; or the switch is turned on. Before the voltage is zero, no loss or interference will occur when the switch is turned on. This turn-on mode is called zero voltage turn-on. In many cases, turn-on or turn-off is no longer indicated. Only zero-current switches and zero-voltage switches are called. The basic circuit is shown in Figure 3 and Figure 4.
Soft-switching circuit control technology is usually adopted, combined with reasonable component layout, printed circuit board wiring and grounding technology, it can improve the EMI interference of switching power supply.
Using electromagnetic shielding measures
Generally, electromagnetic shielding measures can effectively suppress electromagnetic radiation interference of the switching power supply. The shielding measures of the switching power supply are mainly for the switching tube and the high frequency transformer. When the switch tube works, it generates a large amount of heat, and it needs to be equipped with a heat sink, so that a large distributed capacitance is generated between the collector of the switch tube and the heat sink. Therefore, an insulating shielding metal layer is placed between the collector of the switching tube and the heat sink, and the heat sink is connected to the casing, and the metal layer is connected to the zero end of the hot end to reduce the coupling capacitance between the collector and the heat sink, thereby reducing Radiation interference from the heat sink. For high-frequency transformers, the magnet-conducting structure should first be selected according to the shielding properties of the magnetizer. For example, the can-type iron core and the El-type iron core have good shielding effect of the magnetizer. When the transformer is shielded, the shielding box should not be placed close to the transformer, and a certain air gap should be left. If a multilayer shield with air gap is used, the resulting shielding effect will be better. In addition, in high-frequency transformers, it is often necessary to eliminate the distributed capacitance between the primary and secondary coils, and an open-loop ring made of copper foil can be placed between the coils along the entire length of the coil to reduce the disaster between them. In combination, this open circuit ring is connected to the iron core of the transformer and to the ground of the power supply to provide electrostatic shielding. If conditions permit, install a shield on the entire switching power supply, which will better suppress radiated interference.
Conclusion
As the size of switching power supplies is getting smaller and the power density is getting larger, EMI control issues become a key factor in the stability of switching power supplies. It can be seen from the above analysis that EMI filtering technology, shielding technology, sealing technology and grounding technology can effectively suppress and eliminate the interference and radiation between the interference source and the victim device, and cut off the propagation path of electromagnetic interference, thereby improving the switch. Electromagnetic compatibility of the power supply.
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